Conventionally, with the aim of improving the internal quality of continuously-cast slabs, a number of techniques for reducing a slab with a liquid core using reduction rolls installed within a curved type or vertical bending type continuous casting machine (hereinafter referred also to as “liquid core reduction rolling technique”) are proposed. The present inventors also proposed, in Japanese Patent No. 4218383 (hereinafter referred to as “Patent Literature 1”), a continuous casting method of steel, including: reducing a slab with a liquid core in thickness after bulging it, while projecting a lower roll of a pair of reduction rolls above a lower pass line of the slab in a continuous casting machine.
In the liquid core reduction rolling for a slab, molten steel in which elements likely to segregate such as C, Mn, P and S are concentrated (hereinafter referred also to as “segregation-elements-concentrated molten steel”) is discharged to the liquid phase territory by the reduction rolling, whereby compositional segregation in the thickness-wise central part of the slab is improved.
In such a liquid core reduction rolling technique for slabs, if a solidified shell is formed non-uniformly in a width-wise direction of slab, the slab cannot be reduced uniformly in thickness along a width-wise direction. Therefore, the present applicant proposed a method for performing a flow control of molten steel for uniformity of a solidified shell. Concretely, in order to control the geometry along a width-wise direction of slab at a crater end, the present inventors proposed, in Japanese Patent No. 3275835 (hereinafter referred to as “Patent Literature 2”) and Japanese Patent No. 3237177 (hereinafter referred to as “Patent Literature 3”), methods for electromagnetically control flow of molten metal within a mold where the formation of a solidified shell is started.
The method proposed in Patent Literature 2 is a continuous casting method, including: applying a static magnetic field to the cavity of a continuous casting mold in order to obtain an uniform thickness distribution, in a width-wise direction of slab, of a liquid core of a continuously-cast slab at a reduction rolling position, or in order to make thicknesses of width-wise end portions, of the slab to be smaller than that in the width-wise central part of the slab.
The method proposed in Patent Literature 3 is a continuous casting method in which, in order to prevent center segregation, a slab with a liquid core is continuously reduced in thickness while the shape of a solidification line within the slab is controlled so as to decrease the thickness of shell at the central part of the slab by controlling the flow of molten metal continuously supplied into a mold through electromagnetic force of an electromagnetic stirrer located at a distance of 3 to 7 m upstream of a pair of reduction rolls.
The present inventors further proposed, with the aim of controlling the equiaxed structure, continuous casting methods, including: electromagnetically stirring unsolidified molten steel at the upstream site, in the casting direction, relative to a reduction rolling position in Japanese Patent No. 3119203 (hereinafter referred to as “Patent Literature 4”), Japanese Patent Application Publication No. 2005-103604 (hereinafter referred to as “Patent Literature 5”) and Japanese Patent Application Publication No. 2005-305517 (hereinafter referred to as “Patent Literature 6”).
The method proposed in Patent Literature 4 is a method for a liquid core reduction rolling of a slab, including: performing electromagnetic stirring within a mold, further performing electromagnetic stirring of unsolidified molten steel in an unsolidified region of slab with a center solid fraction of 0 to 0.1, and successively imparting an amount of thickness reduction corresponding to 50 to 90% of the thickness of a liquid core by at least a pair of rolls in an unsolidified region of slab with a center solid fraction of 0.1 to 0.4.
The method proposed in Patent Literature 5 is a continuous casting method for reducing a slab with a liquid core, including: electromagnetically stirring unsolidified molten steel at a position in a curved region or bent region of a continuous casting machine where the angle between a tangent line of a circular arc formed by the curved region or bent region and the horizontal plane is 30° or more; installing reduction rolls in a horizontal region of the continuous casting machine at the downstream site from where the electromagnetic stirring is performed: and adjusting, in an area of a slab with a predetermined center solid fraction, the ratio of the amount of thickness reduction D1 to the thickness D2 of a liquid core during reduction to within 0.2 to 0.6.
The technique proposed in Patent Literature 6 relates to a continuous casting method of low-carbon steel, for electromagnetically stirring unsolidified molten steel and reducing a slab with a liquid core located on downstream of the electromagnetic stirring position, including: installing an electromagnetic stirrer at a distance of 3 to 7 m ahead of the most-upstream pair of reduction rolls to apply an electromagnetic force to the unsolidified molten steel so that the ratio of equiaxed structure be 6% or less, and reducing 40% or more of the thickness of a liquid core of the slab therewith, and also relates to a slab cast by the method.
Each of the above-mentioned techniques is the one for controlling the amount of equiaxed structure, existing in a path of discharging molten steel of a liquid core, by means of electromagnetic stirring in order to reduce a slab uniformly in thickness along a width-wise direction thereof and smoothly discharge segregation-elements-concentrated molten steel, and each technique exhibits an excellent effect.
As a result of further studies about a technique for stabilizing center segregation quality of a slab in continuous casting using the liquid core reduction rolling and the electromagnetic stirring, the prevent inventors made clear a problem that as a casting time becomes longer, the segregation-elements-concentrated molten steel as being discharged upstream of the reduction rolling position is enriched much more according to the time and consequently segregated at the tail end of a slab at high concentrations.
FIG. 1 is a view schematically showing the flow of molten steel in the continuous casting involving a liquid core reduction rolling disclosed in Patent Literature 2 or Patent Literature 5. The occurrence of high-concentration segregation at the tail end of a slab, which is the above-mentioned problem, will be described using the same figure.
Molten steel poured into a mold 3 is cooled with spray water injected from the mold 3 and from a set of secondary cooling spray nozzles below it (not shown), and a solidified shell is formed from the outer surface side of the molten steel to yield a slab 8. The slab 8 is withdrawn while a liquid core is present therein, and reduced in thickness by reduction rolls 7 after electromagnetic stirring is imparted to the molten steel of the liquid core by an electromagnetic stirrer 9. The electromagnetic stirrer 9 is generally installed at a distance of 9 m upstream of a meniscus and at a distance of 12 m upstream, in the casting direction, of the reduction rolling position to control the ratio of equiaxed structure.
In the above-mentioned electromagnetic stirring method, the molten steel is caused to flow in a direction from one minor side of slab 8 toward the other minor side thereof while reversing the flowing direction at a predetermined time interval. Such a stirring flow pattern imparted by this electromagnetic stirring method will be hereinafter called “uni-directional alternating flow forming-type stirring”.
In case of the uni-directional alternating flow forming-type stirring, as shown in FIG. 1, the molten steel flows in a major side direction of slab (in a width-wise direction of slab) shown by X1, and this flow runs into the other minor side of slab, whereby there are formed (1) a flow of molten steel directed to upstream in the casting direction in the vicinity of the minor side of slab (shown by f3 and f4 in the figure), (2) a flow of molten steel directed downstream in the casting direction in the vicinity of the minor side of slab (shown by f1 and f2 in the figure) and (3) associated flows of molten steel. The stirring direction of molten steel along a width-wise direction of slab is reversed relative to the direction shown by X1 after the lapse of a predetermined time.
In general, the above-mentioned electromagnetic stirrer 9 is positioned far away from the reduction rolling position, for example, at a distance of 12 m upstream, in the casting direction, of the reduction rolling position, since it is used to control the ratio of equiaxed structure, but not intended to dilute the segregation-elements-concentrated molten steel. Therefore, a stirring force sufficient enough for diluting concentrated elements is not imparted to the segregation-elements-concentrated molten steel, and segregation elements are gradually concentrated in the vicinity of minor sides of slab with the lapse of casting time.
FIG. 2 is a view schematically showing that the enrichment of elements takes place in the vicinity of minor sides at the tail end of slab. The longer the operation time of the continuous casting, the more notable the formation of these elements-enriched zones in the vicinity of minor sides. Therefore, there arise problems that in case of a steel grade which requires further strict control of segregation of elements, it becomes difficult to continue the continuous casting over a longer period of time, and the yield of the slab is reduced.